Abstract
Lubricin (LUB), a major mucinous glycoprotein of mammalian synovial fluids, is believed to provide excellent lubrication to cartilage surfaces. Consequently, when joint disease or replacement leads to increased friction and surface damage in the joint, robust synthetic LUB alternatives that could be used therapeutically to improve lubrication and surface protection are needed. Here, we report the characterization of a lubricating multiblock bottlebrush polymer whose architecture was inspired by LUB, and we investigate the role of fibronectin (FN), a glycoprotein found in the superficial zone of cartilage, in mediating the tribological properties of the polymer upon shear between mica surfaces. Our surface forces apparatus (SFA) normal force measurements indicate that the lubricin-mimetic (mimLUB) could be kept anchored between mica surfaces, even under high contact pressures, when an intermediate layer of FN was present. Additional SFA friction measurements show that FN would also extend the wearless friction regime of the polymer up to pressures of 3.4 MPa while ensuring stable friction coefficients (μ ≈ 0.28). These results demonstrate synergistic interactions between mimLUB and FN in assisting the lubrication and wear protection of ideal (mica) substrates upon shear. Collectively, these findings suggest that our proposed mimLUB might be a promising alternative to LUB, as similar mechanisms could potentially facilitate the interaction between the polymer and cartilage surfaces in articular joints and prosthetic implants in vivo.
Highlights
Successful biomimetic lubricants should prevent wear and reduce friction between contacting surfaces when subjected to (i) high loading pressures, (ii) a wide range of sliding speeds, and (iii) large shearing distances: conditions that are all found in synovial joints
The percent conjugation calculated from the molecular weight (MW) was 83%, the measured polydispersity index obtained from gel permeation chromatography (GPC)-MALLS was Mw/Mn = 1.3, and Mw was 1,400 kDa
Our normal force characterization combined with Alexandre-de Gennes model (AdG) theory (Figure 3) indicates that FN provides a more robust anchoring of mimLUB within the mica junction, as suggested by the larger short-range interaction distance measured for FN + mimLUB films than for FN alone
Summary
Successful biomimetic lubricants should prevent wear and reduce friction between contacting surfaces when subjected to (i) high loading pressures, (ii) a wide range of sliding speeds, and (iii) large shearing distances (with respect to the contact area between surfaces): conditions that are all found in synovial joints Another notable characteristic of synovial joints is their ability to rapidly switch between biolubrication modes, which include boundary and elastohydrodynamic lubrication mechanisms (Swann et al, 1974; Roberts et al, 1982; Jahn et al, 2016). Previous work performed on LUB physisorbed onto mica using the surface force apparatus (SFA, same technique as used in this report) reported very low friction coefficients (μ = 0.02−0.04) when sheared below 0.5 MPa contact pressures, increasing to μ = 0.2−0.6 at higher pressures (Zappone et al, 2007) This remarkable lubrication is believed to arise from the bottlebrush structure of LUB combined with its ability to selfassociate into dimers or multimers that anchor robustly to the cartilage surface (Swann et al, 1985). It has been suggested that, among all aforementioned components, LUB’s highest affinity is for FN (Elsaid et al, 2007)
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